Vehicles and rail cars appeared in late February at Sanumdong, a facility where the North has built some of its largest rockets and missiles.

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The BBC’s Seoul correspondent Laura Bicker says that North Korea might be testing the US after talks in Hanoi between Donald Trump and Kim Jong-un broke down, hoping that the US will offer a better deal to avoid a launch.

Our correspondent adds that experts say the rockets used to launch satellites are usually unsuitable for use as long-range missiles.

A much anticipated meeting between the two leaders in the Vietnamese capital last week ended without a deal over differences in how much North Korea was willing to limit its nuclear programme before it was granted some sanction relief.

What about the Sohae site?

The Sohae launch facility at the Tongchang-ri site has been used for satellite launches and engine testing but never for ballistic missile launches.

This week’s satellite images, coming from several US think tanks and testimony from the South Korean intelligence service, appear to show rapid progress has been made in rebuilding structures on the rocket launch pad.

US National Security Adviser John Bolton has said North Korea could yet face more sanctions if there is no progress on denuclearisation.

A historic first meeting between Mr Trump and Mr Kim in 2018 in Singapore produced a vaguely worded agreement on «denuclearisation» but little progress.